How to add audio to Blender 2D animation?
In the top row you should have 

2D Animation | 2D Full Canvas | Compositing | Rendering | +


Click the "+" in the sub-menu of Video Editing click Video Editing.
In the Video Editing tab navigate to your sound file.
Drag and drop the sound file anywhere into the sequencer.


The sound file is directly in your animation.

Go back to the 2D Animation tab and play your animation to see.